Most cavity wall insulation installs work fine, but problems happen when the job is done badly or in unsuitable properties. The main issue is damp appearing on internal walls after installation. Here's what goes wrong, how to spot it, and what to do if it happens to you.
The most common problem: damp penetration
Cavity walls are designed with a gap (the cavity) to stop rain reaching the inner wall. Water hits the outer brick, runs down the inside face, and the gap prevents it crossing to the inner leaf.
When you fill the cavity with insulation, you risk creating a bridge for moisture. If the insulation is installed unevenly, or if the outer wall has cracks or failed pointing, water can travel across the insulation to the inner wall. That shows up as damp patches, peeling paint, or mould inside your house.
Why damp problems happen
Damp after cavity wall insulation is usually caused by:
Poor installation
Insulation blown in unevenly leaves voids or clumps. Voids don't insulate. Clumps create moisture paths. Both happen if the installer rushes the job or doesn't check cavity width properly.
Unsuitable walls
Properties in exposed coastal areas, houses with cracked render or spalling bricks, and walls with narrow or irregular cavities shouldn't have cavity fill. A good installer surveys first and refuses unsuitable jobs. Bad installers don't survey and just drill.
Blocked weep vents
Weep vents are small gaps in the mortar near the bottom of the wall. They let water drain out of the cavity. If insulation blocks these, water builds up and eventually penetrates inward.
Corroded wall ties
Wall ties are metal rods connecting the inner and outer leaves. If they're rusted, they collect moisture. Adding insulation around corroded ties can make existing damp worse.
How to spot cavity wall insulation problems
Signs that something's wrong:
- Damp patches on internal walls that weren't there before installation, especially after heavy rain
- Mould or musty smell in rooms that were previously dry
- Peeling paint or wallpaper on external walls
- Cold spots on walls where insulation hasn't filled properly
- Increased condensation on windows (sometimes happens because the walls are now colder on the inside surface)
If damp appears within a year of installation, it's almost certainly linked to the insulation. If it appears several years later, it could be insulation-related or it could be a separate issue (roof leak, gutter overflow, rising damp).
What to do if you have problems
- Contact the installer. If the insulation was installed within the past 25 years and you have a CIGA guarantee, contact the installer first. They should investigate and remediate under the guarantee.
- Contact CIGA. If the installer doesn't respond or has gone out of business, contact CIGA (Cavity Insulation Guarantee Agency) directly. CIGA guarantees are insurance-backed, so they'll arrange for another installer to investigate.
- Get a survey. A building surveyor or damp specialist can confirm whether the damp is caused by insulation or something else. Costs around £200 to £400 for a damp survey.
- Keep records. Photograph damp patches, note when they appeared, and keep copies of all correspondence with the installer and CIGA. You'll need these if you make a claim.
Can cavity wall insulation be removed?
Yes, but it's expensive and disruptive. Extraction costs £1,500 to £3,000 for a typical house. The process involves drilling more holes, using a vacuum to suck out the insulation, and then filling all the holes.
Removal makes sense if the damp is severe, the insulation is definitely the cause, and remediation (improving external wall condition, repointing, adding external render) won't fix it.
Most CIGA claims are resolved by improving the external wall condition rather than removing insulation. Removing it is the last resort.
How to avoid problems when installing
If you're planning cavity wall insulation, reduce the risk by:
- Using a CIGA-registered installer. Check their registration number on the CIGA website.
- Insisting on a survey. The installer should assess exposure, wall condition, cavity width, and damp risk before quoting. If they quote based on your postcode alone, walk away.
- Fixing external walls first. Repair cracked render, failed pointing, and damaged bricks before insulating. Don't rely on insulation to fix existing damp.
- Checking your location. If you're within 500m of the coast in an exposed area, or your house is on a hilltop facing prevailing winds, cavity fill is riskier. Ask the installer to check the exposure zone under British Standard BS 8208.
- Getting a CIGA guarantee. Make sure the installer provides a 25-year CIGA certificate. Keep it safe.
Is cavity wall insulation still worth it?
Despite the risk of problems, cavity wall insulation works well in most homes. The vast majority of installs cause no issues. Problems are more common in exposed locations and poorly maintained properties.
If your house is in a low-exposure area, your walls are in good condition, and you use a proper installer, the risk is low and the benefit (£150 to £250 per year saved on heating) is significant.
See our full guide on whether cavity wall insulation is worth it for the complete cost and benefit calculation.
Sources
- Cavity Insulation Guarantee Agency (2025), "Making a claim", ciga.co.uk
- Building Research Establishment (2018), "Cavity wall insulation: performance and risk assessment", BRE Trust